I have had bad luck with it. I get undercut by crazy amounts takes me forever to sell them all

simply buy the ores when they are cheap and sell the gems during peak time (weekends and raiding days).
if your server isnt extremely empty, this will work since the ore prices will skyrocket on peak times and you can still sell them for high profit by undercutting. key is to know how much a gem costs you.
if you however expect to sell all gems on of days by posting them once midday you cannot expect much.

Imo the time spend actually shuffling the ore into sellable materials is time that could be spend making better gold in other sources.

Unless you are farming the ore yourself, or buying it ridiculously cheap, it's not as great a cash cow as it used to be, considering the ease at which you can make gold in other areas of the game.

Remember, time is money, friend.

Also depends on if your enchanter and JC are the same character. IMHO.. the enchanting part of it is gone. Enchanting mats and enchants are just too cheap to be worth it. Probably because of our ability to ´upgrade´ dusts this expansion. In my case, it definitely wasn´t worth sending jewelry to my enchanter.

I am just cutting common gems, selling the perfects and vendoring the normals. The agi rings sell good, so I always keep about 5 of them on hand to sell (ie, I don´t cut the purple gems).

It is a bit random and it will take a few weeks because you can get so many gems (and strictly speaking it cost more since it takes 3 gems for 2 random gems) but if you can farm the Zandalari Warbringers you will get more ore then you can possibly use.